Retirement Benefit Plans (Narrative) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Jul. 01, 2015
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 78
Defined Benefit Plan, Accumulated Benefit Obligation	245,500	$ 227,200
Net actuarial loss	$ 5,100
Assumed healthcare cost trend (as a percent)	7.50%
Healthcare-cost trend rate in 2024 (as a percent)	4.60%
Employer matching contribution, percent of match	100.00%
Employer matching contribution, percent of employees' gross pay	2.00%
Employer matching contribution, additional percent of match	50.00%
Employer additional matching contribution, percent of employees' gross pay	6.00%
Compensation expense	$ 12,000	11,100	$ 10,900
Multiemployer Plans, Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Funded status of plan						$ (800)
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 0	$ 20,000
Expected long-term return on assets (as a percent)	6.50%	7.00%	7.00%
Funded status of plan	$ (13,093)	$ (18,586)
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	122	124
Funded status of plan	(13,096)	(11,806)
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 219	185
Assumed healthcare cost trend (as a percent)	7.50%			8.25%	8.00%
Increase in net periodic postretirement benefit cost if trend increased by 1%	$ 5
Increase in accumulated benefit obligation if trend increased by 1%	148
Decrease in net periodic postretirement benefit cost if trend rate decreased by 1%	5
Decrease in accumulated benefit obligation if trend rate decreased 1%	134
Funded status of plan	$ (3,094)	$ (3,852)
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer matching contribution, percent of employees' gross pay	3.00%